Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/06—Organic material
  • B01D71/66—Polymers having sulfur in the main chain, with or without nitrogen, oxygen or carbon only
  • B01D71/68—Polysulfones; Polyethersulfones
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
  • B01D67/0002—Organic membrane manufacture
  • B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
  • B01D67/0011—Casting solutions therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
  • B01D67/0002—Organic membrane manufacture
  • B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
  • B01D67/0013—Casting processes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/12—Composite membranes; Ultra-thin membranes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/06—Organic material
  • B01D71/08—Polysaccharides
  • B01D71/12—Cellulose derivatives
  • B01D71/14—Esters of organic acids
  • B01D71/16—Cellulose acetate
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/06—Organic material
  • B01D71/30—Polyalkenyl halides
  • B01D71/32—Polyalkenyl halides containing fluorine atoms
  • B01D71/34—Polyvinylidene fluoride
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2323/00—Details relating to membrane preparation
  • B01D2323/12—Specific ratios of components used
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/02—Details relating to pores or porosity of the membranes
  • B01D2325/022—Asymmetric membranes

Definitions

• This invention relates to a process for forming multilayer porous membranes having improved layer to layer junction structure, and to the membrane so formed. More particularly, this invention relates to a process for making a multilayer porous membrane from a multilayered liquid sheet produced by a co-casting method, and to the membranes so formed.
• Polymeric microporous membranes have been prepared previously. Most of the commercialized membranes are symmetric in nature. Symmetric membranes have an approximately uniform pore size distribution throughout the membrane. The production of skinless symmetric microporous membranes are described, for example in U.S. Patent 4,203,848 for polyvinylidene fluoride (PDVF) and in U.S. Patent 4,340,479 for polyamide membranes.
• PDVF polyvinylidene fluoride
• preparations are generally described to consist of the following steps: a) preparation of a specific and well controlled preparation of a polymer solution, b) casting the polymer solution in the form of a thin film onto a temporary substrate, c) coagulating the resulting film of the polymer solution in a nonsolvent and d) removing the temporary substrate and e) drying the microporous membrane.
• Membrane manufacturers produce membranes that are robust and reliable for sterile filtration using such methods. Such membranes are primarily single layer symmetric membranes, although other structures have been investigated and used for such membranes.
• asymmetric membrane Another single layered structure is the asymmetric membrane, where the pore size of the membrane varies as a function of location within the thickness of the membrane.
• the most common asymmetric membrane has a gradient structure, in which pore size increases from one surface to the other. Asymmetric membranes are more prone to damage, since their retention characteristic is concentrated in a thick, dense surface region or skin see US
• U.S. Patent 5,228,994 describes a method for coating a microporous substrate with a second microporous layer thereby forming a two layer composite microporous membrane. This process requires two separate membrane forming steps and is restricted by the viscosities of the polymer solutions that can be used in the process to prevent penetration of casting solution into the pores of the substrate.
• Patent 4,770,777 describes a process with the following steps: casting a first membrane layer, b) embedding a fabric support into this first membrane and c) casting a second membrane layer on top of the embedded fabric to form a kind of membrane/fabric/membrane sandwich.
• U.S. Patent 5,500,167 describes a method of preparing a supported microporous filtration membrane. This method consists of applying a first casting solution onto the first side of a porous nonwoven support material to form a first casting solution layer having a substantially smooth surface, then applying a second casting solution onto the substantially smooth surface of the first casting solution layer to form a second casting solution layer prior to the complete formation of a microporous membrane from the first casting layer, and forming a continuous microporous membrane having first and second zones from the first and second casting solutions such that the first side of the support material is integral with the first zone while not protruding into the second zone, and the first zone has a pore size at least about 50% greater than the pore size of the second zone.
• the inventor has surprisingly found that at essentially no time between the coating applications, one forms membranes in which there is a continuous change of membrane structure without a discontinuity through the junction between layers.
• This demarcation line signifies a drastic change in pore size going from a more open to a more tight structure. It can also signify a region of dense, skin-like structure. Either of these structural regions can lead to a lower permeability and an undesirably fast accumulation of particles at the interface and consequently a drastic flux decline. A more subtle change in pore size between two adjacent would reduce this effect and be beneficial for the retentive behavior of the overall structure of the membrane.
• U.S. Patent 5,620,790 teaches that for sequentially cast membranes, a viscosity restraint is imposed in that the viscosity of the lower layer should be higher than the viscosity of the upper layers.
• a process for producing multilayer microporous membranes including the steps of a) preparing a plurality of lacquers, b) co-casting these lacquers to form a multilayer liquid sheet and c) immersion of the co-cast multilayer liquid sheet into a liquid bath/coagulation bath to effect phase separation in a continuously layer sequence.
• An optional extraction step after coagulation can be provided to extract all the residual solvent within the microporous structure.
• the microporous membrane then is dried under restraint.
• the multilayer microporous membrane of this invention is free of a dense interfacial layer between layers.
• adjacent layers are inseparable and integral with one another, and are free of macrovoids.
• Each of the layers of the multilayer membrane of this invention can be a retentive layer in that they retain a retentate component while permitting through passage of a filtrate.
• Figure 1 is a side view of an apparatus useful in effecting the process of this invention.
• Figure 2 is a graph of a flux of the membrane of this invention as a function of air membrane bubble point.
• Figure 3 is a photomicrograph of a multilayer membrane according to prior art.
• Figure 4 is a photomicrograph of an asymmetric microporous membrane of the prior art.
• Figure 5 is a photomicrograph of an asymmetric membrane of this invention.
• Figure 6 is a photomicrograph of the bottom surface of the membrane of Figure 5.
• Figure 7 is a photomicrograph of the top surface of the membrane of Figure 5.
• Figure 8 is a photomicrograph of a cross-section of a membrane of this invention.
• Figure 9 is a photomicrograph of the top surface of the membrane of Figure 8.
• Figure 10 is a photomicrograph of the bottom surface of the membrane of Figure 8.
• Figure 11 is a photomicrograph of the top surface of an air cast membrane of this invention.
• Figure 12 is a photomicrograph of the bottom surface of an air cast membrane of this invention.
• Figure 13 is a photomicrograph of the top surface of an air cast membrane of this invention.
• Figure 14 is a photomicrograph of the bottom surface of an air cast membrane of this invention.
• Figure 15 shows a cross-sectional microphotograph of a multilayered structure of the present invention wherein both layers are asymmetrical.
• Figure 16 shows a cross-sectional microphotograph of a multilayered structure of the present invention wherein one layer, in this instance the top layer is symmetrical and the bottom layer is asymmetrical.
• the present invention provides for a method of producing an integral multilayered porous membrane by co-casting a plurality of polymer solutions onto a support to form a multilayered liquid sheet and immersing the sheet into a liquid coagulation bath to effect phase separation and form a porous membrane.
• the support can be a temporary support which is removed subsequent to membrane formation. Alternatively, it be incorporated into the final structure if desired After formation, the porous membrane is washed free of solvent and other soluble materials. It can then be further extracted to reduce fugitive materials to a low level and then be dried.
• co-casting means that the individual layers are cast essentially simultaneously with each other with substantially no time interval between one cast layer and the next cast layer. Co-casting is an important aspect of the invention because it allows for formation of controlled pore size regions at the junctions of layers. In the prior art, a well-defined demarcation line is formed between the sequentially cast layers. A drastic change in pore size going from a more open to a more tight structure can lead to undesirable fast accumulation of particles at the interface and/or the formation of a skin layer at the demarcation point and consequently a drastic flux decline.
• a sharp interface can be replaced by a more subtle change in pore size between two adjacent layers.
• Such an interfacial zone is beneficial for the retentive behavior of the overall structure of the membrane. At the same time, it allows the formation of microporous structure with no discemable demarcation line in the structure.
• integral means a structure, that although formed of multiple layers and often different polymeric materials, that is bonded together so that behaves it as one structure and does not delaminate or separate in normal use.
• Figure 1 illustrates a multiple layer forming apparatus 10 for casting multilayered membranes.
• the apparatus is designed to produce a two-layered liquid film and has two chambers 50 and 60 containing the solutions 14 and 16, one for each layer, to be cast. If desired, additional chambers may be added to form additional co-cast layers.
• the apparatus comprises a front wall 20 and a back wall 40 with a separating wall 30 between the front and back walls. The separating wall defines the volumes of the two chambers. Two side walls, not shown, complete the apparatus. In operation, the apparatus is fastened onto a typical membrane casting machine, and a support web 18 is moved or passed under the stationary apparatus and the two solutions are dispensed through gaps or outlets 80 and 90.
• the thickness of the two layers is controlled by the distance (gap) set between the moving web and the outlet, illustrated by gap settings 80 and 90.
• the final liquid layer thickness is a function of gap distance, solution viscosities, and web speed.
• the back wall of the apparatus usually is held a small distance above the support to prevent wrinkling or marring the support. Back wall gap, support speed and solution viscosity are adjusted in practice to prevent solution from leaking out through the back wall gap.
• the apparatus can be fitted with heating or cooling means for each chamber separately, or for the apparatus as a whole, if necessitated by the solution characteristics, or to further control final membrane properties.
• the process relies on gravity driven flow.
• the chambers can be covered and sealed if needed, and fitted with an inlet for a pressurized fluid, usually an inert gas such as dry nitrogen or argon.
• a pressurized fluid usually an inert gas such as dry nitrogen or argon.
• the chambers can then be pressurized separately as required by the solution viscosity and process requirements.
• the solutions are pumped.
• the temporary support 18, such as a non-porous plastic or metal sheet, with the co- cast multilayer liquid sheet is then immersed in a coagulant bath (not shown) as is well-known in the art for a period to effect phase separation of the polymer solutions in a continuously layered sequence and form an integral multilayer microporous polymeric membrane.
• a coagulant bath not shown
• the membrane is usually washed to remove residual solvent and other low molecular weight components of the casting solution and wound onto a core.
• the temporary support 18 is wound onto a separate wind up drum (not shown).
• a nonporous film for uses such as diagnostic strips or a porous, nonwoven fabric such as TYVEK® sheets, a stretched porous PTFE sheet such as is available from W.L. Gore & Associates of Timonium, Maryland, or a microporous membrane made of cellulosic material or plastic or other such materials commonly used in the art as support layers for these types of membranes.
• coagulation occurs from the liquid film surface that first contacts the coagulation bath and then through the subsequent layers of the multilayered liquid sheet.
• Each layer dilutes and changes the coagulant as the coagulant diffuses through the layers.
• Such changes to the nature of the coagulant affect the membrane formation of each layer and of the final multilayer membrane.
• Layer thickness, composition and Jocation of each layer relative to the other layers will affect membrane structure and properties. This is obviously different from a single layer membrane and from membranes made from laminates of single layers. The process is adaptable to complementary membrane formation steps.
• a process of producing symmetric and /or asymmetric membranes in which a single layer liquid film of a membrane forming composition containing a semi-crystalline polymer is briefly heated before phase separation. Such heating controls the final membrane pore size and cross-sectional structure.
• one or several layers would be formed from such compositions utilizing the thermal process of the co-pending application to give additional methods of forming desired pore size and membrane architecture.
• Air casting sometimes called vapor induced phase separation, in which phase separation occurs during an evaporation step, can be adapted to this process by forming a multilayered liquid layer from solutions containing an evaporative solvent and less evaporative non-solvents, and subjecting the liquid film to an evaporative environment, such as a heated air stream, optionally containing water vapor.
• an evaporative environment such as a heated air stream, optionally containing water vapor.
• the process of this invention provides for separate control of the individual regions of the membrane, a region being comprised of membrane formed by each solution and a possible interfacial region and the controlled pore size regions at the junctions of layers.
• This process allows a wider range of useful viscosities of the individual layers, a better control of the thickness of the two or more individual layers and avoids possible skinning effects or dense regions at the interface between the two or more layers, usually between 2 and 4 layers.
• a nonwoven support can be used to increase mechanical stability, although unsupported composite membranes have sufficient mechanical strength (depending on pore size). This combination therefore allows the formation of better controlled, higher integrity, multilayer microporous membranes with improved fluxes.
• the process of this invention also allows for independent casting of very thin layers.
• Layer thickness depends not only on the casting device geometry but also on flow and viscosity of both lacquers.
• the well-defined demarcation line seen in prior art between the two layers can be significantly reduced or avoided.
• a drastic change in pore size going from a more open lo a more tight structure can lead to undesirable fast accumulation of particles at the interface and consequently a drastic flux decline.
• a sharp interface can be replaced by a more subtle change in pore size between two adjacent layers.
• Such an interfacial zone is beneficial for the retentive behavior of the overall structure of the membrane.
• this process can be used with any of the known methods for forming microporous membranes such as liquid casting or air casting. Additionally, the polymers and solvents/nonsolvents used for making such microporous membranes can also be used.
• Preferred polymers include but are not limited to PVDF, nylons such as Nylon 66, polyamides, polyimides, polyethersulphones, polysulphones, polyarylsulphones, PVC, PET, polycarbonates, cellulose, regenerated cellulose, cellulose esters such as cellulose acetate or cellulose nitrate, polystyrenes, polyetherimides, acrylic polymers, methacrylic polymers, copolymers of acrylic or methacrylic polymers, or blends of any of the above and the like.
• the polymer solutions of the present invention typically consist of at least one polymer and at least one solvent for the polymer or polymers.
• the solution may contain one or more components that are poor solvents or non-solvents for the polymer or polymers. Such components are sometimes called "porogens" in the art.
• the solutions are preferably homogeneous. They can optionally contain one or more components, which are non-solvents for the polymer.
• the polymer solution can either be stable in time (good solvent quality) or be meta-stable in time. This solution also can potentially have a lower critical solution temperature or an upper critical solution temperature. Example components of such solutions are well known in the art, and it is not necessary to exhaustively list all possible variations.
• a myriad of porogens have been used in the art, including such examples as formamide, various alcohols and polyhydric compounds, water, various polyethylene glycols, and various salts, such as calcium chloride and lithium chloride.
• the solvents and phase separation materials should be the same if possible or at least compatible so that they do not adversely affect the other layer(s).
• microporous structures of the present invention may have an average pore size of from about 0.01 microns to about 10 microns, preferably from about 0.01 to about 2 microns.
• the use of various formulations and processing steps can create some unique multilayered products.
• Symmetric membranes have a porous structure with a pore size distribution characterized by an average pore size that is substantially the same through the membrane.
• asymmetric membranes In asymmetric membranes, the average pore size varies through the membrane, in general, increasing in size from one surface to the other. Other types of asymmetry are known. For example, those in which the pore size goes through a minimum pore size at a position within the thickness of the membrane. Asymmetric membranes tend to have higher fluxes compared to symmetric membranes of the same rated pore size and thickness. This invention allows one to form unique structures to handle specific applications and needs.
• Asymmetrical membranes may have a pore size gradient of from about 2:1 to about
• This asymmetry is measured by comparing the average pore size on one major surface of the layer with the average pore size of the other major surface of that layer.
• each layer within a wide range and still obtain a self-supporting, integral multilayered structure.
• the thickness of the membrane structure typically be between 50 and 200 microns in thickness as this provides good filtration characteristics and self support.
• the present invention one can still achieve the same overall thickness but can control the relative thickness fo one layer to the other to create unique and desirable membrane structures.
• a first layer that is from about 10 tO about 140 microns thick while the other is correspondingly from about 140 microns to about 10 microns in thickness.
• Figure 15 shows a cross-sectional microphotograph of a multilayered structure of the present invention wherein both layers are asymmetrical.
• Figure 16 shows a cross-sectional microphotograph of a multilayered structure of the present invention wherein one layer, in this instance the top layer is symmetrical and the bottom layer is asymmetrical.
• Solutions were made consisting of 17 to 24 w% polyvinylidenefluoride (PVDF) and 83 to 76% N-methylpyrrolidone (NMP). Each solution was divided into several smaller quantities, which each underwent a different thermal treatment, as shown in Table 1. Each thermal treatment consisted of heating the polymer solution to a predetermined temperature and holding the solution at that temperature for 2 hours. Temperatures ranged from 38 to 50°C. During treatment the solutions were gently agitated using a roller mill. Thereafter the solutions were cooled to room temperature.
• PVDF polyvinylidenefluoride
• NMP N-methylpyrrolidone
• Multilayers of two polymer solutions were co-cast using an apparatus such as illustrated in Figure 1. Lacquer reservoirs 50 and 60 were filled with the two different polymer solutions. By moving the belt relative to the casting apparatus, a multilayer structure was cast without any setting time between the two lacquers. The volumetric flow of the two polymer solutions and the thickness of each layer was controlled by the gap settings 80 and 90 of Figure 1. The formed multilayer was immersed in a methanol bath having a methanol concentration as high as 95% for approximately two minutes. The bath was kept at room temperature. The flow of the methanol was counter current to the direction of introduction of the multilayer to allow good extraction of NMP from the multilayer membrane.
• FIG. 4 shows a dense interface between the two polymer solutions. This membrane was made by sequentially casting a second polymer solution onto a first film of PVDF lacquer. In all of the experiments where lacquers were simultaneously cast, no such interface was observed.
• Two polymer solutions are made using 20% PVDF and 80% NMP.
• a multilayer film was produced by casting a first layer of the PVDF solution on a glass plate. The first layer was exposed to the atmosphere before a second layer was cast on the first layer. Atmospheric exposure was accomplished by having a short space, approximately two centimeters between sequential casting steps. The formed multilayer was immersed into methanol until a membrane was completely formed and then extracted in water. The membrane was dried under restraint in air.
• FIG. 5 shows a typical cross-section of a membrane of the present invention with completely different pore sizes in both layers.
• This membrane was prepared by simultaneously cocasting of two solutions having the same composition (20 w% Kynar 741) and a similar viscosity but a different thermal history. Two regions can be seen, but there is no dense interface.
• a cocast multilayer membrane is produced by casting a solution of higher viscosity for the top layer on a solution of lower viscosity.
• a polymer concentration of 24w% PVDF for the upper layer solution and a polymer concentration of 20 w% PVDF for the lower solution a structure was formed which has a very small pore size at the top part, a larger pore size at the other surface and a zone in the membrane where pore size changes gradually. (See Figure 6). No interface is discemable.
• a 24 % and a 20 w% PVDF solution were made in NMP. Viscosities of a 24 w% and a 20w% PVDF solution are in the range of 12,000 cP and 3,500 cP respectively.
• a multilayer was cast as explained in example 1 , with the 24 w% solution being cast so it faces the methanol bath, the 20 w% solution facing the support belt. SEM images were taken of the multilayer as given in example 3. Surprisingly, no interface was observed between the two layers.
• Figure 6 is a cross-sectional view of the membrane
• Figure 7 is a view of the surface of the membrane at the bath side, showing a skinless tight pore structure
• Figure 8 shows a view of the surface of the membrane at the belt side, showing a skinless open pore structure.
• the first membrane was made by applying solution PS3 on top of PS4, while in the second experiment solution PS4 was on top of solution PS3.
• Figure 12 and 13 show the two surfaces of the first membrane, while figures 14 and 15 those of the second membrane. In both cases, isolated pores are observed in the surface of the membrane that was exposed to air. It is clear that co-casting enables the deposition of two layers of low viscous materials without an intermediate "setting" step. This clearly demonstrates that the proposed process does not need any control over the setting step and is a clear advantage for process controls.
• the inventor has found that co-casting two solutions allows one to control the surface pore size and pore morphology combination for both surfaces.
• the membrane shown in Figures 12 and 13 has a top surface corresponding to the air side surface of a membrane cast from only the solution used for the top surface and a bottom surface formed at the support that corresponds to the bottom surface of a membrane cast only from the polymer solution used to make the bottom layer.
• the example demonstrates that the surface properties of the multilayer membrane can be designed by choosing the formulation properties of the two solutions: i.e. in this example the molecular weight of the polymer,
• Two polymer solutions were made with the following concentrations: 15 w% polyethersulphone, 29.7w% N-methylpyrrolidone and 55.3 w% triethyleneglycol and 11w% polyethersulphone, 32.0 w% N-methylpyrrolidone and 57.0 w% triethyleneglycol. These solutions were cocast according to the present invention. The first solution was used as the bottom solution, while the latter for the top solution. The cocast film was exposed to air for 3.5 sees at a temperature of 69°F and a dew point of 20°F and subsequently immersed in a water bath at 61 °C for about 75 seconds.
• FIG. 15 shows a cross-sectional microphotograph of a multilayered structure of the present invention wherein both layers are asymmetrical.
• Two polymer solutions were made with the following concentrations: 15 w% polyethersulphone, 29.7w% N-methylpyrrolidone and 55.3 w% triethyleneglycol and 13w% polyethersulphone, 14.4 w% N-methylpyrrolidone and 72.6 w% triethyleneglycol. These solutions were cocast according to the present invention. The first solution was used as the bottom solution, while the latter for the top solution. The cocast film was exposed to air for 3.5 sees at a temperature of 68°F and a dew point of 20°F and subsequently immersed in a water bath at 55°C for about 75 seconds.
• the membranes showed a peak IPA bubble point of 29 psi and a water permeability of 1700 Imh/psi.
• Figure 16 shows a cross-sectional microphotograph of a multilayered structure of the present invention wherein one layer, in this instance the top layer is symmetrical and the bottom layer is asymmetrical.

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• 2001
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